Geothermal-nuclear energy release and recovery system

ABSTRACT

A system for mining geothermal energy utilizing the detonation of a deeply buried nuclear device such as nuclear fusion bomb or a nuclear fission bomb to produce a chimney cavity and fractures in a rocky geothermal stratum. Heat exchange fluid is introduced into the cavity and is transferred to flood a higher permeable stratum closer to the surface of the earth. Heat exchangers are introduced into the flooded zone to transfer heat and energy to the surface for utilization.

United States Wtnt [191 Van Huisen 1 @ct. 16, 11973 fifif l fifiifiiffiilt OTHER PUBLCATIONS Non Military Uses of Nuclear Explosives"Scien- Inventor: l n a H 29456 lndlan tific American Dec. 1958 vol. 199No. 6 pp. 29-35.

Valley Rd., Rolling Hill Estates, Calif. 90274 Primary ExaminerCharlesSukalo [22] Filed: Dec. 21, 1970 Att0rneyl\/larvin E. Jacobs Appl. No.:99,898

US. Cl. 165/45, 166/247 Int. Cl. F2811 21/1111 Field of Search 165/1,45; 166/247,

References Cited UNITED'STATES PATENTS 12/1966 Vogel 165/258 10/1969 VanHuisen 165/45 QPE RMEABLE FLOOD ZONE [57] ABSTRACT A system for mininggeothermal energy utilizing the detonation of a deeply buried nucleardevice such as nuclear fusion bomb or a nuclear fission bomb to producea chimney cavity and fractures in a rocky geothermal stratum. Heatexchange fluid is introduced into the cavity and is transferred to flooda higher permeable stratum closer to the surface of the earth. Heatexchangers are introduced into the flooded zone to transfer heat andenergy to the surface for utilization. I

1111 filaims, 3 Drawing Figures ALUVIUM IMPERVIOUS SHALE 58 i 1IMPERVIOUS ZONE CAP ROCK a GEOTHERMAL HOST ZONE PATENTED H81 16 I973IGE'ZOTHERMAL HOST ZONE INVENTOR- ALLEN T. VAN HUISEN hut/MARE 4 ATORNEY GEOTHERMAL-NUCLEAR ENERGY RELEASE AND RECOVERY SYSTEM BACKGROUNDOF THE INVENTION 1. Field of the Invention The present invention relatesto utilization of geothermal energy, and more particularly, to a systemem- I ploying deep detonation of nuclear devices to produce largecavities and fractures for use as geothermal producing zones.

2. Description of the Prior Art The earths geothermal gradient coupledwith the specific heat of the average sedimentary rock demonstrates thatthere is a vast amount of heat energy flowing both towards and parallelto the surface of the earth through the upper few miles of the earthscrust. The source of heat in the upper few miles of the earths crust maybe from the outward flow of heat from the core of the earth, from thecooling igneous magmas, from the disintegration of radioactive elements,from the frictional heat formed during diastrophism, (the rubbingtogether of individual grains), and from the exothermal chemicalreactions that take place within permeable reservoir rocks. 7

The mean heat flow from the interior of the earth to the surface of theearth in continental North America averages approximately 1.2 X 10calorie per centimeter per second. Zones or areas with much higher heatflow are known. A zone approximately 50 to 100 miles wide and severalthousand miles long extends from Easter Island in the Pacific into theGulf of California and on into the southern part of the United States,where the heat flow is to 8 times normal. Substantial areas of the worldare underlayed by rocks of abnormally high temperatures. In many placesheat flow in these regions is as much as 10 times that of the normalcrust.

The average heat flow indicates a temperature gradient in a geologicalregion of average rock type of approximately 1 C per I00 feet of depth.In areas of abnormally high heat flow, the temperature gradient may beas much as 10 C per 100 feet of depth, or more. In areas of hot springactivity or recent volcanic activity, substantially higher temperaturegradients may be found over extensive areas. In many regions,temperatures of as much as 500 C may be found at depths of l0,000 feetor shallower. A substantial amount of energy is stored in such a volumeof hot rock.

In many areas of the earth, deep circulating water is at depths as deepas 10,000 feet or more. The rising heated water carries this heat toshallow depths, heating large volumes of rock. Enormous amounts ofenergy are contained in these masses of heated rock which in some casestotal many cubic miles. The total heat energy in one cubic mile of rockat 10,000 feet depth may be equivalent in energy to 300 million barrelsof oil. However, most of the deep magmatic bodies are not contacted bydeep circulating underground water sources which are essential to raisethe heat to shallower strata from which it can be economicallyrecovered.

Such natural systems in which geothermal reservoirs are in contact witha source of underground water to create geothermal regions withpotentially recoverable heat values are relatively restricted by the lowheat conductivity of the rock and the lack of deep circulating waters.Furthermore, there are very few economical geothermal wells inproduction.

Present utilization of geothermal energy relies on direct thermal fluidmining methods. Geothermally produced gas must be at a sufficientpressure to allow sustained production from the well and the gas mustcontain sufficient energy to drive a prime mover, such as a turbine.Geothermal hot brine must be at a temperature sufficiently higher topermit flashing of the water into steam, which after separation, candrive a steam engine. The remaining brine may be economicallyconvertible into usable commercial salts. The gas, or separated steam,must not contain an undue amount of corrosive salts or gas such asammonia or hydrogen sulfide, and the waste water and brines must bedisposable without polluting surface or potable waters. Furthermore,cavitation, abrasion, scaling and corrosion of equipment must not occurover too short an interval. Moreover, fluid sources of geothermal energyare only available in quite limited parts of the earth and are thus notgenerally available for heat or energy utilization.

Geothermal reservoirs which lack fluid can only transmit heat byconduction. Furthermore, many geological strata are too impermeable topermit adequate invasion of a heat transfer fluid through the hotgeothermal strata in order to mine and recover the heat values throughwells drilled into the strata. The conductivity to heat is very low innon-porous strata, whereas, the capacity to transmit heat is higher inbeds with higher permeablity or in wet geothermal areas as isexperienced in conventional geothermal steam producing areas. However, aheavily fractured and cavernous formation is practically a true boilerin which the resistance to movement of heat is reduced.

There have been recent proposals to utilize energy released by theunderground explosion of a nuclear device or to utilize the combinedgeothermal and nuclear energy conserved within the chimney formed by theexplosion. ln one proposal, a nuclear device is introduced into apre-formed cavity in an isolated, sub-terranian, compact, competent,geological formation, such as a salt formation, and it is then actuated.The energy liberated-by the detonation is retained within the zone andwater is then introduced into the zone and directly transferred to thesurface for utilization. In another proposal, a detonation occurs in ahot geothermal strata to form a heavily fractured and cavernousformation containing large amounts of porous rubble which functions as atrue boiler. Again water is introduced into the chamber to convert it toheated water or steam which is directly piped to the surface.

It has been demonstrated that nuclear fusion or fission devices can bedetonated underground in competent geological strata in which the weightof the earth will serve to contain the explosion. The explosion willfracture and cavitate a substantial area of the adjacent rock to form arubble cone pro-tecting both the heat source from the detonation andfrom the fractured geothermal rock. Such detonations are attendent withmany difficulties.

One of the dangers relates to the distribution of radioisotopes withinthe cavernous and fractured area after the explosion which cancontaminate the introduced heat exchange fluid. Without specialprecautions, such radioactive elements are a hazard to the surface areaswhere the heat exchange fluid is utilized. Radioactive shielding andcontrol devices are both expensive and the devices are of limitedreliability. This greatly adds to the cost of mining and utilization andprecludes the selection of such a system as a pollutionfree, low costsource of energy. Another problem is that the cost of detonation is veryhigh and the area of use is limited to adjacent the detonation site.Therefore, the final cost of energy may not be economical or of suchgreat cost that only a very large user could consider such aninvestment.

SUMMARY OF THE INVENTION In accordance with this invention, thegeothermal heat energy and the energy released by a nuclear explosionare more efficiently and economically recovered without the need totransport radioactively contaminated fluids to the surface of the earth.Furthermore, the invention permits distribution and utilization of thegeothermal energy over a wide area by both small users and by industrialusers. Geothermal strata containing substantial heat energy, but lackingmoisture and permeability, are rendered usable to substantiallycontribute to energy requirements now being satisfied mainly by fossilfuels.

The geothermal-nuclear energy distribution system in accordance with theinvention, includes a cavity formed in a hot geothermal strata by meansof a nuclear detonation, means for introducing a heat exchange fluid tothe cavity to transfer the geothermal and heat energy generated by thenuclear device to the fluid, means for transporting the heated fluid toa higher porous sub-surface zone for flooding and heating the zone; andheat exchange means having a closed end inserted into the flooded porousstrata and having an open end and means communicating said open end withthe surface of the earth for indirectly delivering said geothermal heatto the surface for utilization.

To recover the geothermal energy from the deep hot strata, a nucleardevice is introduced into the strata and detonated to form a fluidpermeable cavity. A heat exchange injection well is drilled into thecavity from the surface and a heat exchange transfer well is drilledfrom the cavity to a higher permeable zone. The permeable zone isflooded with heat exchange fluid from the transfer well. Heat recoverywells are then drilled from the surface into the flooded zone and closedend heat exchangers implanted at the bottom of the heating wells forindirect recovery of geothermal energy.

The invention will now become better understood by reference to thefollowing detailed description when considered in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a systemfor recovery and distribution of geothermal-nuclear heat energy;

FIG. 2 is a schematic view illustrating the application of the inventionto distribution of the energy over a large area such as a city fortapping by many users; and

FIG. 3 is a section taken along line 33 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thesystem according to the invention includes a nuclear produced cavity 10,a heat exchange fluid injection system 12 for introducing heat exchangefluid into the cavity 10, a heat exchange transporting conduit 14 fortransporting the fluid to a near-surface permeable zone 16, andoptionally, a recirculation system 18 for removal of fluid from zone 16and returning to cavity 10. Structures 20 overlying the zone 16 floodedwith the heat exchange fluid may tap into the zone 16 with a closed endheat exchanger 22.

The cavity 10 is formed in a geothermal heat host zone 24 having a meantemperature of at least 200 C and preferably, from about 500 to 800 C.The composition of the host zone should be selected with a view toforming a highly permeable rubble cone. A nuclear detonation in a salt,limestone or shale formation would be expected to result in substantialcompaction of the rubble after formation and some loss of permeabilitythrough the rubble cone with time. However, if the nuclear shot weremade in basalts, metamorphic rocks or granite-rock type in a region ofhigh heat flow, one can expect the rubble cone to stay open andpermeable for almost indefinite periods. The latter rock types arecommon in regions of high heat flow. Another usually coincidentgeological condition is the presence of a cap rock strata above the hotgeothermal layer.

This is important since the detonation must be contained to avoid escapeof radioactive gases, to contain the thermal energy produced by thenuclear blast and furthermore, to prevent collapse and caving at thesurface of the earth. Containment is achieved if the internal cavitypressure is equal to or less than the overburden or lithostatic pressureat the time the shock front wave has been reflected from the surfaceback to the cavity wall. Experience to date indicates that hard rocklayers support the detonation without significantly increased groundsupport problems. Furthermore, available experimental data from thePlowshare program permits the prediction of cavity size, chimney heightand tonnage of broken rock for underground nuclear explosions withconsiderable confidence.

For a specified rock type, predicted accuracy is within 10 percent.Criteria for establishing minimum depths of burial to prevent dynamicventing have also been established. For explosions in hard rock, suchdynamic venting can be prevented by a depth of burial equal to theanticipated chimney height plus a 300 to 500 foot thick buffer ofoverlying rock cover.

The cavity 10 is produced by drilling a hole to implant the nucleardevice at a shot point 26 in the geothermal host zone 24. The device isdetonated to produce the cavity 10 by the expansion of the explosionproduced gases. The lower cavity boundary is characterized by a meltrock interface 28, while the cavity fills with a chimney of broken rock30, resulting from gravity collapse of the cavity. A zone of fracturedrock 32 immediately surrounds the chimney.

A S-megaton shot detonated at a depth of 10,000 feet in rock having ageothermal temperature of approximately 500 C can be expected to producea cavity approximately 500 feet in radius. This cavity will have avolume of approximately 5 X 10 cu. ft. The cavity will collapse in a fewseconds after its formation and a rubble cone extending upwardly towardsthe surface of the earth is formed by successive caving actions. If theporosity of the rubble is approximately 12 percent, a volume of rubblein the cone equal to approximately eight times the volume of the cavitywill be formed. This rubble cone will extend upward to approximately2,000 feet from the surface of the earth. There will be approximately 4X 10 cu. ft. of rubble in the cone and the mean temperature of therubble will be approximately 350 C.

Water introduced into the stop of the rubble cone should flashimmediately to steam. The specific gravity of the rock can be assumed tobe about 2.5 and its specific heat to be about 0.25. At temperaturesabove 100 C, approximately 4.5 X calories are available from each cubicfoot of rock. Thus, there are 1.8 X 10 calories of energy available insuch a rubble cone. This contrasts with the heat liberated by theexplosive which is approximately 5 X 10 calories. In summary, the amountof heat available is approximately five times the energy of theexplosive, and this heat can be produced at a constant controlled ratesimply by the controlled injection of water into the recovery cone.Approximately 10 pounds of super-heated steam can be produced from theenergy of the rubble.

The cost of drilling a 30 inch diameter hole to 10,000 feet is estimatedto be not in excess of4 million dollars. The service charge of thedevice to be detonated will be approximately 1 million dollars. Thus, itappears that the cavity will be economic by a factor of at least 2.

The geological formation selected for the detonation in addition tohaving a competent lower geothermal host zone 24 also has a geologicallycompetent upper geological stratum 16. The upper formation 16 is astratum which is selected to have a geological capacity to receive thethermal flood which occurs when the fluid heated in the lower fracturedformation is delivered to it. The hot fluid, or steam, leaves the cavity10 by means of the heat exchange transporting conduit 14. The conduit 14may be located in the drill site utilized to implant the nuclear device.The heat exchange conduit 14 contains perforations 34 in the portiontraversing the permeable flood zone 16. The heated fluid permeatesthrough and floods the zone 16, and is capable of maintaining the zoneat a temperature of at least 100 C, preferably at least 150 C. Theconduit 14 should be cased with a low thermal conductivity material toprevent heat losses as the fluid is raised to the zone 16.

The permeable flood zone 16 should be disposed between two impermeablegeological zones to restrict and contain the transferred heat exchangefluid. The overlying zone 36 can be an impermeable shale rock. Theunderlying zone 38 may be formed of some other impermeable geologicalstrata. The zone 16 should be located a minimum distance from thesurface, suitably about 200 to 500 feet, so that the cost to theindividual user of drilling a well for implanting a heat exchanger 22within the zone is at a minimum cost. Depleted oil and gas fieldsusually have competent properties for a permeable flood zone and couldbe used for the heat flood storage zone when they are in the proximityof utilization.

A second bore hole or well 42, is drilled from the surface 44 of theground into the cavity 10 at a location near the top of the rubble 30for injection of heat exchange fluid, suitably water, into the cavity.The heat exchange fluid is introduced in a controlled manner from awater storage tank 46 by means of pump 48 when valve 50 is opened. Thebore hole 42 is preferably located a distance away from the cavity 10 sothat pump 48 can also be used to pump the heated exchange fluid out ofthe zone 16 through a stand-by heat exchange recovery well 54 when valve56 is open. The

heat exchange fluid is then re-injected into the rubble cone through theinjection well 42. A second recovery well 58 may be drilled into thepermeable heat flood zone 116 on the other side of transfer conduit 14.A second recirculation well 60 may be drilled from the surface 44 nearthe recirculation well 58, having a lower end communicating with theZone 110 near the top of the rubble 30. On actuation of pump 62, theheat exchange fluid will be drawn out of the zone 16, through withdrawalwell 58 and re-injected into the rubble cone 110 through recirculationwell 60.

Thus, the natural convection movement of the heat exchange fluid upwardthrough pipe 141 and relatively outward through the perforations 34 intothe zone 16 is materially aided by activation of pumps 48 and 62 towithdraw the heat fluid from the formation 16 through the recovery wells54 and 58. The resultant movement of the heat exchange fluid in theformation 16 greatly increases the efficiency of the heat exchange withthe heat exchangers22 submerged therein. The recovery and reinjection'operations can be activated during times of peak energydemand. Duringminimum demand periods, excess heat exchange fluid may be transportedthrough transfer conduit 14 to the surface either naturally byconvection or aided by means of a pump 66. The heat exchange fluid canbe utilized during these minimum demand periods to operate a turbine 68and electric generator 70 or other suitable uses. The condensate fromthe turbine 68 can be returned to the formation 16 by means of a recyclewell 711.

The quality and quantity of the heat exchange fluid being generated atany instant can be sensed at the top of well 114 by means of a gauge 72.The signal, from the gauge 72 can be applied to a controller 74- whichcan operate and actuate the various valves 50 and 56 and the pumps 48,66 and 62.

Each of the deep wells 42, 1 .4 and 60 are provided with suitablepressure-control means, fluid monitoring means and such other equipmentthat is necessary to insure the efficient and trouble free operation ofthe wells for the intended purpose. The withdrawal and reinjection offluid from the flood stratum 116 in addition to providing movement ofthe fluid through the flood zone and improving heat exchange efficiencyalso prevents saturation of the flood zone with the liquid. The locationof the recycle wells at the periphery of the intended boundary ofutilization also can be utilized to scavenge the fluid and return it tothe lower cavity and thus prevent permeation and waste of the heatexchange fluid through extended areas not intended to be utilized forheat or energy exchange purposes.

The flooded zone 16 acts as a storage zone for the energy injected intoit via the circulation of heated water and/or steam. The storing of heatenergy upon the surface of the earth is both difficult and expensive.The present invention permits the transfer of geothermal heat energyfrom a deep strata into a higher storage subsurface zone where it isabsorbed into the formation and retained for future use. Furthermore,the system l6 and insert a closed-end heat exchanger 22 at the bottomthereof. The radiant heat may be piped to the surface through the well80 but preferably boiler grade water is fed by gravity from storagereservoir 82 into the heat exchanger 22 by means of pipe 84 and steam isreturned to the surface through conduit 86 preferably assisted by meansof pump 88 which also pumps the steam into the distribution system 90for heating the space enclosed by building 20. The heat well 80 and heatexchanger 22 may be constructed and operated according to my previousUS. Pat. No. 3,470,943 entitled Geothermal Exchange System. A series ofheat exchange wells 80 and heat exchangers 22 may also be utilized toprovide the energy for the heat irrigation system disclosed in myprevious US. Pat. No. 3,521,699, entitled Earth Energy ConservationProcess and System."

The geothermal-nuclear energy release system can be expanded inoperational scope so as to be able to flood large areas servingcommunities and cities. Referring now to FIGS. 2 and 3, a plurality ofnuclear detonated cavities are created in a pattern underneath the city100. In a city of the size of 30,000 to 60,000 population, it isestimated that four rubble cones 102 provided at each corner of the citywould have sufficient capacity to flood the zone 16 and providesufficient heat for space heating and energy requirements for the city.Each cavity 10, is provided with a heat transfer conduit 104 havingperforations 106 within the permeable zone 108. A withdrawal well 110,pump 112 and recycle well 114, is associated with each cavity 10. Eachpump 112 may also be connected to a reservoir of feed water 115 througha line 116 containing a valve 118.

The rubble cones 102 are prepared by nuclear detonation as describedabove, and, the recirculation wells 114 are then drilled into thecavities 102. The withdrawal wells 110 are then drilled into the porousformation 108 and the pump 112 is connected to the withdrawal well 110and the recycle well 114 at each corner of the city. Feed water 115 isintroduced to the rubble tions 106 and into the layer 108. Thewithdrawal wellsh 110 maintain the fluid moving through the thermallyflooded permeable layer 108 when valve 119 is open and valve 118 isclosed. Ground users such as home owners, business owners and municipalusers may simply tap and utilize the thermal energy within zone 108 bydrilling heat wells 120 from the surface into the heat flooded zone andinstalling a closed bottom heat exchanger 122 at the end of the heatwell 120. It is predicted that the amortization of the installation forsuch a community system will be at such a low rate, that users may becharged a fixed fee for tapping into the zone to heat their house ratherthan requiring metering and charge by BTU or calorie unit.

The thermal energy system of the invention will conserve conventionalenergy and fossil fuels and will obviate the need to burn pollutionproducing for energy production fuels near large population centers. Thesystem of the invention prevents radiation hazard since the heat ismined by indirect heat-exchange. Economic feasibility is clearlyindicated based upon available data on heat energy sales and the cost ofsteam production.

The present invention when practiced in competent geological formationsto provide containment of the explosion permits economically tapping theheat energy released into the cavity by the explosion and thecontinuously transferring and storing the energy in a higher permeableflood zone.

lt is to be realized that only preferred embodiments of the inventionhave been disclosed and that numerous substitutions, alterations andmodifications may be readily made by those skilled in the art withoutdeparting from the spirit and scope of the invention as defined in thefollowing claims. What is claimed is: l. A goethermal-nuclear energyrecovery and distribution system comprising in combination:

a fractured, highly permeable, rubble cone cavity formed in a geothermalstrate having high heat flow and a mean temperature of at least 200 C bythe detonation of a nuclear device within the cavity; a cap rock stratedisposed above the cavity to contain radioactive gases and thermalenergy and to prevent collapse and caving at the surface of the earth;

injection well means extending from the surface of the earth to thecavity for introducing'heat exchange fluid into the cavity and totransfer the geothermal and heat energy generated by the nuclear deviceto the fluid;

transfer well means extending from the cavity through the cap rockstrata to a higher, fluid permeable, sub-surface zone for flooding andheating the zone with the heated fluid;

an upper and lower impermeable geological zone disposed above and belowthe heat flooded zone to restrict and contain the transferred heatexchange fluid within the flooded zone;

heat recovery well means extending from the surface into the floodedzone; heat exchanger means having a closed end received within theterminus of the recovery well and extending into the heat flooded,permeable zone; and

means for delivering a second heat exchange fluid to the heat exchangerand for conducting the heated second fluid to the surface of the earthfor utilization.

2. A system according to claim 1 in which said subsurface permeable zoneis no deeper than 500 feet from the surface and further including meansfor circulating heated fluid through said permeable sub-surface zone.

3. A system according to claim 2 in which the circulating means includesa plurality of said cavities disposed at spaced points underlying asurface community, each of said cavities being provided with aninjection well and a transfer well.

4. A system according to claim 2 in which said circulating meanscomprised at least one withdrawal well extending from the surface intothe permeable subsurface zone for withdrawing fluid therefrom and atleast one reinjection well extending from the surface into the cavityfor receiving the withdrawn fluid and for reinjecting the withdrawnfluid into the cavity.

5. A system according to claim 1 in which the geothermal strata has amean temperature of from 500 C to 800 C and the strata is formed of amaterial selected from basalt, metamorphic and granite rock types.

6. A system according to claim 1 further including pump means associatedwith said well means.

9. A system according to claim 1 in which the transfer well means islined with an impermeable conduit extending from said cavity at least tosaid fluid permeable zone and said conduit contains perforationsadjacent said permeable zone.

10. A system according to claim 9 in which the conduit is lined with lowthermal conductivity material.

11. A system according to claim 1 in which the outlet of said injectionwell means into said cavity is located near the top of the cavity.

UNITED STATES PATENT OFFICE QERTIFIQATE OF CORRECTION Patent No. 3 ,765,477 D t d October 16 1973 Inventor(s) Allen '1. Van Huisen It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 2," line 30, "permeablity" should read -permeabilityline 57,"pro-tecting" should be --protecting-. Column 8, line 14, "goethermal"should read "geothermal"; line 17, "strate" should read -strata--; line20,- "strate" should 'read --strata-.

Signed and sealed this26th day of March 1974.

(SEAL) Attest:

EDWARD M.FLETCHER,JR, C. MARSHALL DANN Attesting Officer Commissioner ofPatents FORM PO-IOSO (10-69) Usc OMM-DC 60376-P69 \a n 0.5. oovzmmimrnmnuc OFFICE: ls" o-su-azu

1. A geothermal-nuclear energy recovery and distribution system comprising in combination: a fractured, highly permeable, rubble cone cavity formed in a geothermal strate having high heat flow and a mean temperature of at least 200* C by the detonation of a nuclear device within the cavity; a cap rock strate disposed above the cavity to contain radioactive gases and thermal energy and to prevent collapse and caving at the surface of the earth; injection well means extending from the surface of the earth to the cavity for introducing heat exchange fluid into the cavity and to transfer the geothermal and heat energy generated by the nuclear device to the fluid; transfer well means extending from the cavity through the cap rock strata to a higher, fluid permeable, sub-surface zone for flooding and heating the zone with the heated fluid; an upper and lower impermeable geological zone disposed above and below the heat flooded zone to restrict and contain the transferred heat exchange fluid within the flooded zone; heat recovery well means extending from the surface into the flooded zone; heat exchanger means having a closed end received within the terminus of the recovery well and extending into the heat flooded, permeable zone; and means for delivering a second heat exchange fluid to the heat exchanger and for conducting the heated second fluid to the surface of the earth for utilization.
 2. A system according to claim 1 in which said sub-surface permeable zone is no deeper than 500 feet from the surface and further including means for circulating heated fluid through said permeable sub-surface zone.
 3. A system according to claim 2 in which the circulating means includes a plurality of said cavities disposed at spaced points underlying a surface community, each of said cavities being provided with an injection well and a transfer well.
 4. A system accoRding to claim 2 in which said circulating means comprised at least one withdrawal well extending from the surface into the permeable subsurface zone for withdrawing fluid therefrom and at least one reinjection well extending from the surface into the cavity for receiving the withdrawn fluid and for reinjecting the withdrawn fluid into the cavity.
 5. A system according to claim 1 in which the geothermal strata has a mean temperature of from 500* C to 800* C and the strata is formed of a material selected from basalt, metamorphic and granite rock types.
 6. A system according to claim 1 further including pump means associated with said well means.
 7. A system according to claim 6 further including heat sensing means for sensing the heat content within the flooded subsurface permeable zone and for developing a signal indicative thereof and control means connected to said sensing means for selectively actuating said pump means.
 8. A system according to claim 4 including a plurality of withdrawal wells located near the periphery of the boundary of surface utilization and reinjection wells connected to said withdrawal wells for reinjecting the heated heat exchange fluid into the cavity.
 9. A system according to claim 1 in which the transfer well means is lined with an impermeable conduit extending from said cavity at least to said fluid permeable zone and said conduit contains perforations adjacent said permeable zone.
 10. A system according to claim 9 in which the conduit is lined with low thermal conductivity material.
 11. A system according to claim 1 in which the outlet of said injection well means into said cavity is located near the top of the cavity. 